Electrically driven implantable devices are used principally as cardiac pacemakers, but they have also been considered for heart assist systems, drug infusion and dispensing systems, defibrillators, nerve and bone growth stimulators, gut stimulators, pain suppressors, scoliosis treatment apparatus, artificial vision apparatus, artificial hearts, artificial larynxs, bladder stimulators, brain stimulators, muscle stimulation, and implanted sensors.
The basic pacemaker system consists of an electrode attached to the heart and connected by a flexible lead to a pulse generator. This generator is a combination of a power source and the microelectronics required for the pacemaker system to perform its intended function. A fixed rate pacemaker provides continuous pulses to the heart, irrespective of proper heart beating, while a demand inhibited pacemaker provides pulses only when the heart fails to deliver a natural pulse. Depending upon the various sensed events, the pacemaker stimulates the right atrium, the right ventricle, or both chambers of the heart in succession. The pacemakers in current use incorporate circuits and antennae to communicate noninvasively with external instruments called programmers. Most of today's pacemakers are of the demand inhibited type, hermetically sealed, and programmable.
Early pacemakers were powered by primary zinc-mercuric oxide cells. Although this system was used for about 15 years, it did suffer from high self-discharge and hydrogen gas evolution. Several mechanisms contributed to battery failure, most of them related to the chemistry of the cell. In addition, the open-circuit voltage of each cell was only 1.5 V per cell and several cells had to be connected in series to obtain the required voltage for pacing. Furthermore, the pacemaker could not be hermetically sealed due to the gas evolution, and had to be encapsulated in heavy epoxy. In 1970, the average life of the pulse generator was only 2 years, and 80 percent of explants were necessitated by failed batteries.
Consideration was given to many means of power generation and power storage. This included primary chemical batteries of all sorts, nuclear batteries, rechargeable batteries, and the separation of the stimulator system into two parts, with the power pack being outside the patient's body and transmitting pulses of energy to a passive implanted receiver and lead. Cardiac pacemakers based on rechargeable nickel-cadmium systems (1.2 V per cell) and rechargeable zinc-mercuric oxide systems (1.5 V per cell) were developed. Such pacemakers are described in prior art references, including U.S. Pat. Nos. 3,454,012; 3,824,129; 3,867,950; 3,888,260; and 4,014,346. The rechargeable pacemaker incorporated a charging circuit which was energized by electromagnetic induction, or other means. This produced a current in the charging circuit which was made to flow to the rechargeable battery. Although this system was incorporated in many cardiac pacemakers, it was unpopular among patients and physicians primarily because the frequency of the recharges was too high (weekly), and the nickel-cadmium system suffered from memory effects which reduced the battery capacity exponentially after each recharge. In addition, the specific energy density of both types of rechargeable batteries was poor, the call voltages were low, there was no clear state-of-charge indication, and hydrogen gas liberated during overcharge was not properly scavenged either through a recombination reaction, or hydrogen getters.
One of the problems in charging a nickel-cadmium cell or a zinc-mercuric oxide call is that they have a fairly flat voltage-time curve, and hence poor state-of-charge or discharge indication. Overcharged nickel-cadmium cells liberate oxygen exothermically at the nickel electrode which migrates to the cadmium electrode and recombines to form cadmium hydroxide. In some cases, this is a poor charge indication since for fast charge, the rate of oxygen evolution may be higher than the rate of oxygen recombination, and cells may not reach full charge, leading to an excess of gas pressure and cell venting. The overcharge reaction involves a slight heating of the cell which lowers its cell voltage. It is this -.DELTA.V which is used quite commonly in commercial nickel-cadmium chargers to determine full charge of nickel-cadmium cells.
Other means of controlling the recharging operation have also been used. For example, U.S. Pat. No. 3,775,661 teaches that the pressure buildup internally can be sensed by a diaphragm that is external to the battery. As the pressure within the cell casing increases, the diaphragm is flexed to actuate an associated switch connected in the battery charging circuitry, deenergizing the charger when the battery internal pressure indicates a fully charged state.
U.S. Pat. No. 4,275,739 uses a diaphragm internal to the cell and the deflection of this diaphragm during internal pressure increase indicates the cell reaching full charge.
U.S. Pat. No. 3,824,129 suggests using a "stabistor" connected in parallel to the cell such that when the cell voltage approaches or reaches the fully charged level, the stabistor diverts the charge current, and stops call charging. This approach does not lead to efficient means of charging or provide adequate safety for nickel-cadmium or zinc-mercuric oxide calls since both cells exhibit a fairly flat charge voltage and the end-of-charge indication is poor. Hence, cells may evolve gas which may go undetected before the stabistor diverts the current.
In U.S. Pat. No. 3,942,535 an implanted cardiac pacemaker is described in which the charging current is monitored and a signal, whose frequency is related to the current amplitude, is transmitted external to the patient. The patent further describes an electrical shunt regulation in the charging circuit to prevent excessive voltage and current from being applied to the rechargeable cell. In U.S. Pat. No. 3,888,260 similar means are described. These patents do not control the amount of charging and the problems of gas evolution described above may arise.
U.S. Pat. No. 4,082,097 describes a system for controlling the charge of a battery, and a battery protection device designed to sense the state of charge and limit the charging amplitude. A pressure switch is incorporated whose function is to provide a signal to the circuitry when the battery reaches a preselected charge state. In addition, an undefined electrode is incorporated that provides an output voltage whose amplitude is related to the state of charge of the battery. The function of this electrode is to sense any parameter that changes with the state of charge.
The only rechargeable systems available at the time of these patents were those based on conventional chemistries, such as nickel-cadmium, lead-acid, etc., and these cells only evolve gas close to their full charge levels. Hence, the pressure switch could only be functional at or close to the charged state. Furthermore, since gas evolution was not a true function of the state of charge, the electrode incorporated to provide a voltage change or sense some parameter could not also be functional.
Today, most nickel-cadmium chargers utilize different means to control and cease battery charging. The end-of-charge indicators for such calls can be maximum voltage, maximum time, maximum temperature, a negative .DELTA.V, dV/dt, .DELTA.T, or dT/dt. The details of these end-of-charge indicators can be found in EDN, May 13, 1993.
Both zinc-mercuric oxide or the nickel-cadmium cells, the previous battery system for rechargeable battery heart or body stimulators, suffered from problems such as memory effects, and high self-discharge. Fast recharge could only be accomplished if the battery was fast-charged to some preselected voltage followed by a trickle charge. It was well known that nickel-cadmium batteries that are fast charged cannot be charged to 100 percent of rated cell capacity. This loss of capacity is due to what is called a memory effect. Each time the battery is discharged at some low current rate, and charged at a higher current rate, the loss in capacity accumulates. Such cells then have to be fully discharged and "reconditioned" before the full capacity can be recovered. Because of this capacity loss and also loss due to high self-discharge, the patient had to have the pacemaker recharged every week. The rechargeable battery pacemakers were developed for lifetimes of over 10 years. Yet, because of the battery chemistry, they only lasted 2 or 3 years, which was the same lifetime as that of primary cells at that time.
It is desirable therefore, to incorporate a battery system in a bioimplantable device such as a cardiac pacemaker or a defibrillator, that overcomes many of the previous problems. The desirable battery parameters should include a high cell voltage, long cycle life, high discharge rate capability, high charge rate capability, no memory effect, no gas evolution, non-toxic chemicals in the battery, high energy density, ability to shape the battery in various configurations, low self-discharge, proper state-of-charge indication, and improved reliability. In addition, the charge control of the battery needs to be properly controlled in order to prolong the life of the battery and to minimize recharge time.